The present invention relates generally to a switched reluctance motor, and more particularly, to a switched reluctance motor having reduced noise and vibration levels.
Generally, a switched reluctance motor (SRM) is a simple motor having a very high rotating speed. However, SRM's are very limited in their application because they typically experience excessive noise and vibration. In a switched reluctance motor, a main excitation force is an electromagnetic force applied in a radial direction between the stator and the rotor. The electromagnetic force is determined by the inductance of the motor and the electrical current passed through the coil of the motor. Several methods are known for reducing the noise and vibration levels in a motor. One such method concerns reducing the power component of the excitation force in the motor body by modulating current wave shapes of the electric power which drives the SRM. In this case, the power component to be reduced has a frequency adjacent to a dominant natural frequency of the motor body. However, this method does not provide adequate means to reduce the component of an excitation force in a SRM having a high rotating speed. Another known method is to make a switch dither at the time the switch is turned on or off such that the energy from the excitation force component is dispersed to surrounding frequencies of the component. In the case where the excitation force component has a frequency remote from the dominant natural frequency, this method is not applicable for the high rotating speeds of the SRM.
In yet another known method, the current wave shape, at the time the switch is turned off, is modulated such that the power of the excitation force component is reduced. However, in this case, the noise and vibration levels at the time the switch is turned off are not always the most significant contributors to the noise and vibration level. For example, for motors with large torque and power levels, the switching angle at turn on is set to get away from the angle of overlap between to tooth portion of a rotor with a tooth portion of a stator. In this case, the above method fails to compensate for the noise and vibration generated in the overlap angle.
Meanwhile, in case of a motor having a relatively low dominant natural frequency, noise is generated not only at the time a switch is turned off, but also at the time an edge of a rotor tooth begins to meet an edge of a stator tooth.